UICollectionViewCell sometimes not showing up - ios

Question

I'm trying to get collection view cells with a background image and text on them. When I build and run it sometimes they show up in the simulator and sometimes they don't even when I don't change the code.
- (void)viewDidLoad
{
[super viewDidLoad];
self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayout alloc] init];
CGRect screenRect = [[UIScreen mainScreen] bounds];
CGFloat frameWidth = screenRect.size.width - 40;
CGFloat frameHeight = screenRect.size.height - 20;
_collectionView=[[UICollectionView alloc] initWithFrame:CGRectMake(20, 20, frameWidth, frameHeight) collectionViewLayout:layout];
[_collectionView setDataSource: self];
[_collectionView setDelegate: self];
[_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:#"cellIdentifier"];
[_collectionView setBackgroundColor:[UIColor clearColor]];
[self.view addSubview:_collectionView];
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return [self.articles count];
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"cellIdentifier" forIndexPath:indexPath];
UILabel *articleTitle = [[UILabel alloc]initWithFrame:CGRectMake(0, 0, cell.bounds.size.width, 40)];
NSDictionary *article = self.articles[indexPath.row];
NSData * imageData = [[NSData alloc] initWithContentsOfURL: [NSURL URLWithString: article[#"image"]]];
cell.backgroundView = [[UIImageView alloc] initWithImage:[UIImage imageWithData:imageData]];
articleTitle.text = article[#"title"];
articleTitle.textColor = [UIColor whiteColor];
[cell.contentView addSubview:articleTitle];
return cell;
}
"articles" is a json object with title, image and url keys. They seem to work when the cells show up but as I said they don't always show.
This is my first app so any help would be really appreciated.

Answer 1

The numbers of section will be depend on article count.
Try placing breakpoint in there, your article array might be empty, if so collection view data source will not get any data to be displayed.
Or if self.articel refers to an array then try replacing line
NSDictionary *article = self.articles[indexPath.row];
With
NSDictionary *article = [self.articles objectatIndexPath:indexPath.row];
Or post your full code so we could see for other possible bug.